Transmission of pathological data patterns

ABSTRACT

A method and apparatus is presented for reducing or eliminating pathological data patterns from signals for transmission over optical communications systems. One embodiment includes a decoder/deserializer configured to receive an encoded serial digital signal, a ditherer configured to dither a least significant bit of each digital data word, and a reserializer/encoder configured to serialize digital data and encode it, for example according to an applicable communication standard such as SMPTE 259M. The improvements may be provided in a single removable unit, such as a small form-factor pluggable (SFP) module compatible with existing optical communications equipment.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/712,335 filed Aug. 29, 2005, the contentsof which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to devices, systems, and methods of datacommunication and, more particularly, to data communication of digitalvideo signals.

BACKGROUND OF THE INVENTION

Communication of digital data, especially digital video data, can be ademanding enterprise. The desire for improved performance has ledinvestigators to attempt alternative methods and modes of communicatingdigital video. As a result, a wide variety of data communicationstechnologies, including equipment, standards, protocols, etc., have beendeveloped with varying performance characteristics.

One standard for transporting digital video data is known as SMPTE 259M,developed by the Society of Motion Picture and Television Engineers(SMPTE) for communication over coaxial cables. This standard describes amethod for use with switched or dedicated interconnecting cables withone signal per cable confined to relatively short distancecommunication, for example within a building or campus environment.

Recent applications of the SMPTE 259M standard have indicated a need toextend its use to conditions differing from those originallycontemplated. For example, users have indicated a desire to employ thestandard for data communication over longer distances, over alternativephysical media such as optical fibers, and without the limitation of onesignal per cable. Recent improvements in fiber optic communicationssystems have spurred interest in combining SMPTE 259M signals with othersignal types in a multi-service fiber backbone, for example a densewavelength division multiplexed (DWDM) backbone.

One problem with communicating SMPTE 259M signals over opticalcommunications systems involves the content of the signals and how thatcontent affects the performance of certain optical devices employed inthe system. For example, certain signals known as “pathological signals”can adversely affect the power control circuitry of optical transmittersand/or receivers and increase signal-to-noise ratio, bit error rate,intersymbol interference, and/or other adverse effects. One exemplarypathological signal includes a repeating pattern of digital bits inwhich one bit (either high or low) is followed by nineteen consecutivebits of the opposite polarity, i.e. 01111111111111111111 or10000000000000000000. Such a pathological signal may include significantlow frequency content and may disrupt system devices not designed forlow frequency signals.

Optical transmitters commonly include power control circuitry that mayattempt to keep the average optical output power at a predeterminedlevel to compensate for degradation of the laser threshold over time andtemperature. The power control circuitry responds according to a longtime constant relative to the modulation waveform of the transmitteddata signal, essentially using the DC portion of the modulation spectrumfor laser output power control. If a transmitted data signal includessignificant low frequency content, the transmitted waveform may bedistorted by the power control circuitry, as illustrated in FIG. 2.

FIG. 2 is a simplified plot 200 of time versus optical output power foran exemplary optical transmitter driven with low frequency data. Theoptical transmitter's power control circuitry tends to increase ordecrease the optical output power toward the average optical outputpower 206 over time, thus distorting the waveform at peak output power204 (e.g. corresponding to long runs of high bits or ones) and at loweroutput power near the laser threshold 202 (e.g. corresponding to longruns of low bits or zeros). Such waveform distortions may cause the biterror rate and picture quality at the receiver to suffer.

Optical receivers and transmitters often used in optical communicationssystems are commonly designed under an assumption that the signal beingtransmitted is a 50% duty cycle modulating (e.g. AC-balanced) signalover some period of time such as 1 μs. For digital video data, anexemplary pathological signal including the repeating pattern with 1:19ratio of ones to zeros described above may continue for up to about 50μs, which of course fails to conform to the design assumption. Duringsuch pathological signals, the average optical output power may increaseor decrease, significantly distorting the transmitted waveform andpossibly causing overmodulation and/or adversely affectingsignal-to-noise ratio. Overmodulation of the optical transmitter maycause additional waveform distortion and ringing, leading to intersymbolinterference as the laser drive current moves into nonlinear regions ofthe light versus current (LI) curve such as that shown in FIG. 1.

FIG. 1 is a simplified plot 100 of laser drive current versus opticaloutput power for an exemplary optical transmitter. As the drive currentdecreases from its level at peak optical output power 104 to the laserthreshold 102, overmodulation can result in spectral broadening of thelaser output as the laser acts more like an LED, for example when thedrive current approaches the “knee” of the LI curve (e.g. near the laserthreshold 102). In this way, waveform distortion can result inintersymbol interference which in turn may cause increased bit errorrate, corruption of video data, and/or degradation of picture quality.

Therefore, there is a need to solve the above-described problemsassociated with transporting data including pathological signals overoptical communications systems.

SUMMARY OF THE INVENTION

In light of the foregoing the inventors have concluded that there is aneed for an improved method of communicating data signals over opticalcommunications systems in which the adverse effects of pathologicalsignals are reduced or eliminated. The inventors have recognized thatmany, if not all, of the described adverse effects of pathologicalsignals may be avoided by “dithering,” or adding a small amount of noiseto the least significant bit of each data word in parallel form. Suchdithering may break up the repeated nature of pathological signals, thusavoiding any associated adverse effects.

The inventors have realized that dithering may be applied in addition toencoding prescribed by applicable communication standards such as SMPTE259M. Accordingly, the inventors have appreciated that a signal encodedaccording to an applicable communication standard such as SMPTE 259M maybe decoded and de-serialized, the resulting parallel digital data signalmay have its least significant bit dithered, and serialization andencoding steps according to the standard such as SMPTE 259M may bere-applied, in preparation for transporting the signal over one or moreoptical communications systems including a multi-service fiber backbone.

Likewise, the inventors have realized that certain types of ditheringare reversible, permitting mathematically-equivalent recovery of theoriginally encoded signal on the receiver end of an opticalcommunications system. Accordingly, the inventors have appreciated thata transported signal encoded according to an applicable communicationstandard such as SMPTE 259M may be decoded and de-serialized, theresulting parallel digital data signal may have its least significantbit un-dithered, and serialization and encoding steps according to thestandard such as SMPTE 259M may be re-applied, thereby restoring thetransported signal to its originally encoded state, including anypathological data patterns.

As will be described below, the inventors have developed variousembodiments of the invention according to these and other discoveries.According to one embodiment of the invention, an improved device isprovided for preparing signals (e.g. including any pathological datapatterns) for transmission over optical communications systems,including a decoder/deserializer configured to receive an encoded serialdigital signal, a ditherer configured to dither a least significant bitof each digital data word, and a reserializer/encoder configured toserialize digital data and encode it, for example according to anapplicable communication standard such as SMPTE 259M. One embodiment ofthe invention provides the improved device in a single removable unit,such as a small form-factor pluggable (SFP) module compatible withexisting optical communications equipment.

An exemplary embodiment of the invention for use on the receiving end ofan optical communication system includes a decoder/deserializerconfigured to receive a serial digital signal encoded for exampleaccording to an applicable communication standard such as SMPTE 259M, anun-ditherer configured to un-dither a least significant bit of eachdigital data word, and a reserializer/encoder configured to receive andserialize digital data and encode it, for example according to thestandard such as SMPTE 259M, thereby restoring the signal to itsoriginal state (e.g. including any pathological data patterns) forfurther transportation and/or processing. One embodiment of theinvention provides the improved device in a single removable unit, suchas a small form-factor pluggable (SFP) module compatible with existingoptical communications equipment.

In an optical communications system, the improved devices may be usedboth for preparing signals for transmission on the transmitting end andrecovering signals equivalent to the originally encoded signals on thereceiving end. One embodiment of such a system includes adecoder/deserializer, ditherer for dithering a least significant bit,and a reserializer/encoder in the transmit path, as well as adecoder/deserializer, un-ditherer for un-dithering a least significantbit previously dithered, and a reserializer/encoder in the receive path.One embodiment of such a system includes each of these receive-path andtransmit-path devices on the transmitting end of a multi-service fiberbackbone, and additional receive-path and transmit-path devices on thereceiving end of the multi-service fiber backbone.

A method according to one embodiment includes receiving a serial digitalsignal encoded for example according to an applicable communicationstandard such as SMPTE 259M (e.g. including any pathological datapatterns), decoding and de-serializing the digital signal, dithering aleast significant bit, and re-serializing and encoding the digitalsignal for example according to SMPTE 259M. One embodiment of theinvention includes receiving a serial digital signal encoded for exampleaccording to SMPTE 259M that has been transported over an opticalcommunications system, decoding and de-serializing the digital signal,un-dithering a least significant bit, and re-serializing and encodingthe digital signal for example according to SMPTE 259M thereby restoringa signal equivalent to its original state (e.g. including anypathological data patterns) for further transport and processing.

When employed in the context of optical communications, embodiments ofthe present invention significantly improve transmission of signalsencoded for example according to an applicable communication standardsuch as SMPTE 259M (e.g. including any pathological data patterns) overadvanced optical communications networks including, for example, amulti-service DWDM fiber backbone. Methods and devices embodying theseadvantages may be provided in a form suitable for use with conventionaldata transmission networks and at a reasonable cost.

The present invention together with the above and other advantages maybest be understood from the following detailed description of theembodiments of the invention illustrated in the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates a simplified plot of laser drive current versusoptical output power for an exemplary optical communications device;

FIG. 2 illustrates a simplified plot of time versus optical output powerfor an exemplary optical communications device;

FIG. 3 illustrates a simplified schematic diagram of an exemplary pseudorandom sequence generator in accordance with one embodiment of theinvention;

FIG. 4 illustrates a simplified schematic diagram of an exemplarydithering configuration in accordance with one embodiment of theinvention;

FIG. 5 illustrates a simplified schematic diagram of an exemplaryconfiguration for processing a serial digital signal in accordance withone embodiment of the invention;

FIG. 6 illustrates a simplified schematic diagram of an exemplaryconfiguration for reversing signal processing of FIG. 5 in accordancewith another embodiment of the invention;

FIG. 7 illustrates a simplified schematic diagram of an exemplarydecoding and deserialization configuration in accordance with oneembodiment of the invention;

FIG. 8 illustrates a simplified schematic diagram of an exemplaryserialization and encoding configuration in accordance with oneembodiment of the invention;

FIG. 9 illustrates a simplified schematic diagram of an exemplaryoptical communications system in accordance with one embodiment of theinvention;

FIG. 10 illustrates an optical communications system formed inaccordance with another embodiment of the invention; and

FIG. 11 shows a flow diagram illustrating an exemplary method ofprocessing a digital video signal in accordance with one embodiment ofthe invention.

DETAILED DESCRIPTION

The following description is provided to enable a person of ordinaryskill in the art to make and use the disclosed inventions and sets forththe best modes presently contemplated by the inventors for carrying outtheir inventions. In the following description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the described inventions. It will beapparent to one skilled in the art, however, that the inventions may bepracticed without these specific details. In other instances, structuresand devices are shown in block diagram (or otherwise simplified) formfor clarity of presentation.

In the following discussion, the singular term “signal” and plural term“signals” are used interchangeably and are to be understood as includinganalog or digital information, at a single frequency or a plurality offrequencies, and may or may not include coding, modulation, sidebandinformation, or other features of signals or waveforms well known in theart. Furthermore, when reference is made to a “receiver,” “transmitter,”“output,” or “input,” previous process steps may have been utilized toform signals or waveforms compatible with these features. In addition,no particular order is required for the method steps described below,with the exception of those logically requiring the results of priorsteps, for example re-serializing a signal may logically require theprior de-serializing of that signal. Otherwise, enumerated steps areprovided below in an exemplary order which may be altered, for instancethe several processing steps may be rearranged or performedsimultaneously.

One or more embodiments of the invention will be described withreference to the SMPTE 259M standard for transporting serial digitalinterface (SDI) uncompressed digital video data. However, the inventionand its various embodiments are not so limited, and the invention may beemployed in conjunction with other types of data and in conjunction withother standards, means, and modes of data communication including, forexample high-definition digital video data communications in accordancewith the SMPTE 292M standard.

The various SMPTE standards for transmitting digital video data overserial data links were developed in part with the goal of permittinggreater flexibility and ease of communicating such data between users.As networking technology has advanced, users have naturally tended to“push the envelope” and attempt to apply the standards in contextsbeyond those originally contemplated.

The expanding infrastructure of high-speed data networks developed bythe telecommunications industry has enticed users with large amounts ofbandwidth and a relatively economical cost structure. Users looking totransport digital video data over longer distances, to new places,and/or in a more flexible environment have investigated application ofthe existing serial digital video standards to such data networks. Theseusers have encountered a number of problems.

Among the problems that have been encountered include those related tothe content of the signals being transmitted. It is well known that thetransmission of digital data signals over networks can be improved byencoding the digital data to, for example, reduce the low frequencycontent of such signals. Standards for transmitting digital video data,including for example the SMPTE 259M standard, prescribe that areversible transformation or encoding be applied to the serial datastream in order to guarantee transitions of at least one bit in eachdigital word transmitted.

As an example, the SMPTE 259M standard prescribes that a scrambler ofthe type shown in simplified schematic form in FIG. 3 should be appliedto the serial digital video data stream. Referring to FIG. 3, ascrambler 300 may be formed with D flip-flops 310 and exclusive OR (XOR)gates 308 configured as shown. A feedback shift register formed from theD flip-flops 310 includes feedback taps strategically placed to generatea maximum-length pseudo-random number sequence when employed with theXOR gates 308 as shown. A clock input 304 is included to coordinateoperation of the scrambler 300 according to the desired frequency.

In operation, the serial data stream is applied at the data input 302and a clock signal is applied at clock input 304, resulting in ascrambled serial data stream at the data output 306. For a knowninitialization state of the D flip-flops 310, scrambling of the serialdata stream at the data output 306 is reversible, permitting recovery ofthe input serial data stream using a reciprocal configuration as is wellknown in the art.

For the scrambler 300 as shown in FIG. 3, the scrambling polynomial isG(X)=(X ⁹ +X ⁴+1)(X+1)which includes both a pseudo-random sequence generator (X⁹+X⁴+1) and anon-return-to-zero (NRZ) to non-return-to-zero inverted (NRZI) converter(X+1).

To avoid transportation of clock signals in addition to data signals,the scrambling technique employing scrambler 300 may beself-synchronized, so that a clock signal must be generated at thereceiver device that is equivalent to that at the transmitter, forexample clock recovery using a phase locked loop (PLL) device. BecausePLL devices in common use may be particularly sensitive to data errors(e.g. caused by alignment jitter or phase differences between therecovered clock and input signal, bit misidentification, etc.), suchdata errors should preferably be minimized during transmission andreception of the transported data signal.

Although the scrambler 300 effectively guarantees transitions betweenhigh and low (i.e. 1 and 0) states at the output for each digital dataword, serial digital video data may present additional problems. Othertypes of digital data ordinarily produce an uncorrelated signal, inwhich each digital data word is unrelated to the previous digital dataword. Rather, it is highly unlikely that the same digital data word orseries of digital data words would repeat or recur in the exact samesequence for a prolonged period.

However, serial digital data, unlike other types of digital data, oftenproduces a highly correlated signal. A serial digital video signal mayexhibit the same bit pattern repeated over and over, for example for anentire active picture scan line of digital video or for an entire frameor sequence of frames. A highly correlated signal of this type mayproduce one or more pathological signals that are not eliminated by ascrambling technique such as that prescribed by SMPTE 259M and describedabove with reference to FIG. 3.

For a highly correlated signal, a scrambler 300 of the type shown inFIG. 3 may fail to eliminate the adverse effects of pathologicalsignals. For a 10-bit data word, the guaranteed transition may occuronly in one bit, resulting in a long run of high bits, e.g., 0111111111to 1111111111, or low bits, e.g. 1000000000 to 0000000000. When repeatedmany times, the signal may continue to exhibit low frequency content,possibly resulting in the previously mentioned data errors andcorruption.

In response to this problem, the Society of Motion Picture andTelevision Engineers (SMPTE) has provided an Engineering Guideline,EG34, suggesting that a small amount of noise be added to the serialdigital video signal to eliminate the repeating pathological patternwithout compromising the integrity of the serial digital video datasignal. One arrangement for implementing this guideline is illustratedin FIG. 4.

FIG. 4 shows a configuration 400 for “dithering,” or adding apseudo-random sequence of transitions, to the least significant bit ofeach digital data word. The dithering is applied to the video data inparallel form, and thus prior to any scrambling such as that prescribedby SMPTE 259M, for example.

Referring to FIG. 4, digital video data in parallel form is received atthe input 402. The most significant bit (MSB) of each digital data wordis shown at the top of input 402 and the least significant bit (LSB) isshown at the bottom of input 402. The exemplary configuration 400includes D flip-flops 410 and exclusive OR (XOR) gates 408 arranged in afeedback shift register with feedback taps for forming a pseudo-randomsequence generator with scrambling polynomial:G(X)=X ⁵ +X ²+1A clock input 404 is included to coordinate operation of theconfiguration 400 according to the desired frequency. In operation, theconfiguration 400 introduces noise in the least significant bit of thevideo data signal, effectively breaking up the repeating nature of thepathological signal. The dithering is applied to the least significantbit (LSB) as shown in FIG. 4, such that a dithered digital video datasignal in parallel form is provided at the output 406.

Similar to the scrambler 300 illustrated in FIG. 3, for a knowninitialization state the dithering introduced using the configuration400 is reversible, for example at the receiving end of an opticalcommunications system, permitting recovery of the input data patternusing a reciprocal configuration as may readily be understood by aperson skilled in the art.

Dithering of the type described in FIG. 4 may be applied to a serialdigital video data stream as shown in FIG. 5. FIG. 5 shows a signaldithering device 500 in block diagram form, according to one embodimentof the invention. FIG. 5 illustrates the signal dithering device 500 fordithering a serial digital data stream previously encoded according to aknown standard, for example the SMPTE 259M standard, including adecoding/de-serialization block 508, a parallel data bus 514, adithering block 510, and an encoding/re-serialization block 512.

In operation, a serial digital video signal is received at input 502. Inaccordance with this embodiment of the invention, the serial digitalvideo signal has previously been scrambled according to a knownstandard, for example SMPTE 259M, to permit effective data transmissionor processing via coaxial cable or other means (not shown), but maynevertheless include pathological data patterns. The serial digitalvideo signal received at input 502 is unscrambled in accordance with theknown standard and de-serialized (e.g. converted from serial to parallelform) in the decoding/de-serialization block 508. The resulting videodata signal in parallel form has its least significant bit dithered inthe dithering block 510, as described above with reference to FIG. 4.The dithered parallel video data signal is then re-serialized (e.g.converted from parallel to serial form) and encoded according to theknown standard in the encoding/re-serialization block 512. At the output506, the encoding/re-serialization block 512 provides the resultingdithered serial digital data stream (e.g., without pathological datapatterns). Alternate embodiments of the invention may permit decodingand encoding in the blocks 508, 512 according to a variety of standardsand arrangements, as should be apparent to those skilled in the art.

FIG. 6 illustrates a device 600 for un-dithering a serial digital datastream previously dithered, such as a serial digital stream dithered inthe manner shown and described with reference to FIG. 5. FIG. 6 shows asignal un-dithering device 600 in block diagram form according to oneembodiment of the invention, including a decoding/de-serialization block608, a parallel data bus 614, an un-dithering block 610, and anencoding/re-serialization block 612.

In operation, a serial digital video signal is received at input 602. Inaccordance with this embodiment of the invention, the serial digitalvideo signal has previously been dithered (e.g. as shown in FIG. 5) andscrambled according to a known standard, for example SMPTE 259M, topermit effective data transmission or processing via coaxial cable orother means (not shown). The serial digital video signal received atinput 602 is unscrambled in accordance with the known standard andde-serialized (e.g. converted from serial to parallel form) in thedecoding/de-serialization block 608. The resulting video data signal inparallel form has its least significant bit un-dithered in theun-dithering block 610, thereby reversing the dithering described abovewith reference to FIGS. 4 and 5. The un-dithered parallel video datasignal is then re-serialized (e.g. converted from parallel to serialform) and encoded according to the known standard in theencoding/re-serialization block 612. At the output 606, theencoding/re-serialization block 612 provides the resulting serialdigital data stream (e.g. including any pathological signal patterns).Again, alternate embodiments of the invention may permit decoding andencoding in the blocks 608, 612 according to a variety of standards andarrangements, as should be apparent to those skilled in the art. Invarious embodiments of the invention, the device 600 for un-ditheringmay be functionally identical or complementary to the signal ditheringdevice 500.

FIG. 7 illustrates an exemplary configuration 700 for thedecoding/de-serialization blocks 508, 608 (see FIGS. 5, 6) in accordancewith one embodiment of the invention. The configuration 700 includes aninput serial data bus 702, a decoder 708, a serial coupler 704, ade-serializer 710, and an output parallel data bus 706.

In operation, a serial digital data stream encoded according to a knownstandard, for example SMPTE 259M, is received at the input serial databus 702 and descrambled at the decoder 708. The decoder 708 reverses theencoding prescribed by the known standard to produce an unscrambledserial digital data stream. The decoded serial digital data stream isconverted to parallel form at the de-serializer 710, for example using ademultiplexer as is well known in the art, and output at the outputparallel data bus 706.

FIG. 8 illustrates an exemplary configuration 800 for theencoding/re-serialization blocks 512, 612 (see FIGS. 5, 6) in accordancewith one embodiment of the invention. The configuration 800 includes aninput parallel data bus 802, a re-serializer 808, a serial coupler 804,an encoder 810, and an output serial data bus 806.

In operation, a parallel digital data stream is received at the inputparallel data bus 802 and converted to serial form at the re-serializer808, for example using a multiplexer as is well known in the art. Theencoder 810 encodes the serial digital data stream according to a knownstandard, for example SMPTE 259M (see FIG. 3), to permit effective datatransmission or processing. The resulting encoded serial digital datastream is provided at the output serial data bus 806.

FIG. 9 shows an optical transmission system 900 utilizing dithering toeliminate pathological data patterns in accordance with anotherembodiment of the invention. The optical transmission system includes atransmitter device 908, one or more fiber optic pathways 904, and areceiver device 910. The transmitter device 908 includes an input 902adapted to receive a serial digital video data signal encoded accordingto a known standard, for example SMPTE 259M, a signal dithering device912 such as the signal dithering device 500 shown and described withreference to FIG. 5, and an optical communications amplifier 918 andoptical source 916. The receiver device 910 includes an optical receptor920 and optical communications amplifier 922, an un-dithering device 914such as the device 600 shown and described with reference to FIG. 6, andan output 906 adapted to provide a serial digital video data signalencoded according to the known standard employed in transmitting device908 or some other standard or encoding method desired by a user.

The optical communications amplifier 918 and optical source 916 may beany combination suitable for use in optical communications transmission,for example a laser diode or light-emitting diode (LED) and compatibleamplifier configured for use with optical fibers or other form of fiberoptic pathway 904. Likewise, optical receptor 920 and opticalcommunications amplifier 922 may be any combination suitable for use inoptical communications reception, for example an avalanche photo-diode(APD) and transimpedance amplifier (TIA) configured for use with opticalfibers or other form of fiber optic pathway 904.

Of course, a person skilled in the art would readily appreciate thattransmitter device 908 and receiver device 910 may include additionaldevices or components (not shown). For example, devices for furtherprocessing and conditioning of serial digital video data signals may beincluded upstream and/or downstream of devices 912, 914 in one or bothof the transmitter and receiver devices 908, 910, for the purpose ofincorporating the serial digital video signals into a multi-service datastream transported using a DWDM backbone.

FIG. 10 illustrates a digital video data communications system 150utilizing dithering to eliminate pathological data patterns inaccordance with another embodiment of the invention. The digital videodata communications system 150 includes a source/server for digitalvideo data such as a studio 152, a server-side data pathway 160, atransmitter device 156, one or more fiber optic pathways 166, 168, areceiver device 158, a client-side pathway 162, and a client/consumer ofdigital video data such as a post-production facility 154.

The transmitter device 156 includes at least one removable unit 164, forexample a small form-factor pluggable (SFP) module known to becompatible with a variety of optical communications equipment. Theremovable unit 164 includes a signal dithering device 500 (see FIG. 5)configured to accept an input serial digital interface (SDI) datasignal. Likewise, the receiver device 158 includes at least oneremovable unit 170 including a signal un-dithering device 600 (see FIG.6) configured to reverse dithering applied at the signal ditheringdevice 500 of the removable unit 164.

In operation, the digital video data communications system 150 functionsto transport digital video data between the source/server 152 and theclient/consumer 154. Raw digital video data may be serialized andencoded according to a known standard, for example SMPTE 259M, at thesource/server 152 for transmission via server-side data pathway 160, forexample at least one coaxial cable. The resulting serial digital videodata signal is transported via pathway 160 to the removable unit 164 attransmitter device 156. At the removable unit 164, a signal ditheringdevice 500 of the type illustrated in FIG. 5 decodes and de-serializesthe input serial digital data signal, dithers a least significant bit ofthe digital video data in parallel form, and re-serializes and encodesthe digital video data signal in accordance with the known standardchosen at the source/server 152. In this way, any pathological datapatterns that may be present in the digital video data signal at thetransmitter device 156 are eliminated to permit more advantageoustransmission of the digital video data signal using the one or morefiber optic pathways 166, 168. After dithering, the digital video datamay be subjected to further processing and possible multiplexing withother signals for transmission using the fiber optic pathways 166, 168,which may include, for example, a DWDM multi-service backbone.

At the receiver device 158, optical signals including the transmitteddigital video data signal are received and may be subjected to furtherprocessing and possible demultiplexing. The digital video data signalsare provided to the removable unit 170, at which a signal un-ditheringdevice 600 of the type illustrated in FIG. 6 decodes and de-serializesthe digital video data signal, un-dithers a least significant bit of thedigital video data in parallel form, and re-serializes and encodes thedigital video data signal in accordance with the known standard chosenat the source/server 152. In this way, the serial digital video datasignal is restored to its state as originally received at removable unit164, including any pathological data patterns that were present in thedigital video data signal on the source/server side. The resultingserial digital video data signal is transported via pathway 162 (e.g.including at least one coaxial cable) to the client/consumer 154, wherethe raw digital video data may be recovered by decoding andde-serialization in accordance with the known standard chosen at thesource/server 152.

A person skilled in the art would readily appreciate that, althoughtransport of digital video data in a single direction (i.e. fromsource/server 152 to client/consumer 154) is shown and described withreference to FIG. 10, video data may be communicated in either directionin accordance with this or other embodiments of the invention. Those ofskill in the art would understand that fiber optic pathways 166, 168 asillustrated in FIG. 10 are not required, and various embodiments of theinvention may use additional or substitute pathways for wired, wireless,optically coupled, repeating, transponding, media converting, switching,multicasting, WDM trunk, CWDM, OADM, and/or other types of datacommunication techniques. For example, serial digital video data may betransported via a link including free space optics (FSO) componentsbetween transmitter device 156 and receiver device 158. For anotherexample, serial digital video data may be transported via a linkincluding physical layer switching components between one or moretransmitter devices 156 and a plurality of receiver devices 158.

FIG. 11 shows a flow diagram representation of a method 700 of ditheringa digital video data stream to eliminate pathological data patterns inaccordance with another embodiment of the invention. The method 700includes a receiving step 254, a decoding/de-serialization step 256, adithering step 258, a re-serialization/encoding step 260, and aprovision step 262.

The method 700 begins at step 252 and proceeds to the receiving step 254in which a serial digital signal is received that has previously beenencoded according to a known standard, for example SMPTE 259M. Thereceived serial digital signal may be a serial digital video signalincluding pathological data patterns that may hinder or preventsuccessful communication of the signal via certain opticalcommunications systems. For example, given a signal dithering device 500of the type illustrated in FIG. 5, the encoded serial digital signal maybe a serial digital video signal such as a serial digital interface(SDI) signal received at input 502. The receiving step 254 may beaccomplished via coupler to a coaxial cable or some other method ofreceiving wired or wireless signals well known in the art.

The method 150 then proceeds to decoding/de-serialization step 256, inwhich the received serial digital signal is decoded and de-serialized togenerate a parallel digital signal. Decoding is performed to reverseencoding previously applied according to the known standard, for exampleSMPTE 259M. For example, given a decoding and de-serializationconfiguration such as configuration 700 illustrated in FIG. 7, decodingof the received serial digital signal may be performed in the decoder708 to generate a decoded serial digital video data stream. In thisexample, decoding may be performed to reverse encoding applied using ascrambler 300 of the type shown in FIG. 3. Continuing this example,de-serialization may then be performed in the serial-to-parallelconverter 710, for example a demultiplexer, thus generating a decodedparallel digital signal.

The method 150 proceeds to dithering step 258, in which an amount ofnoise is added to the parallel digital signal. This noise is added bydithering a least significant bit (LSB) of each digital data wordaccording to a reversible dithering technique. For example, given asignal dithering device 500 of the type shown and described withreference to FIG. 5, a dithering block 510 may apply dithering via acoupler for the least significant bit of the decoded parallel digitalvideo signal. In this example, a dithering configuration 400 of the typeshown in FIG. 4 may be employed to apply noise to the least significantbit of each digital data word using a pseudo random sequence generatorformed from a feedback shift register, feedbacks taps, and XOR gates.Although a five-bit pseudo random sequence generator with scramblingpolynomial: G(X)=X⁵+X²+1 is used in this example, such a scramblingtechnique is not required, and any reversible scrambling techniqueamenable to dithering a LSB of a parallel digital signal may be used asshould be apparent to those skilled in the art.

The method 250 then proceeds to re-serialization/encoding step 260, inwhich the dithered parallel digital signal is converted from parallel toserial form and encoded in accordance with the known standard previouslyemployed. For example, for a signal dithering device 500 of the typeshown in FIG. 5, encoding/re-serialization block 512 may be used toconvert the parallel digital signal into a serial digital signal, forexample using a multiplexer, and the serial digital signal may beencoded using a scrambler 300 of the type shown and described withreference to FIG. 3. In this example, the encoded serial digital signalis a serial digital video data signal encoded according to the SMPTE259M standard.

The resulting serial digital signal may be provided for furtherprocessing and transport in provision step 262. Due to the small amountof noise added to the signal in dithering step 258, any pathologicaldata patterns present in the serial digital signal received in receivingstep 254 have been eliminated. The resulting serial digital signalformed at re-serialization/encoding step 260 is suitable for use withoptical communications systems including one or more fiber opticcommunication networks employing a multi-service DWDM backbone. Forexample, given a serial digital video data signal containingpathological data patterns received at receiving step 254, the method250 may provide a resulting serial digital video data signal devoid ofsuch pathological data patterns to be used in place of the receivedsignal. Of course, the above described method or its reciprocal forun-dithering may be used on both ends of an optical communicationssystem for effective transport of signals including pathological datapatterns, as should be readily apparent to those skilled in the art. Inthis way, the previously mentioned bit errors, data errors, PLLalignment jitter, and the accompanying adverse effects on picturequality may be reduced or avoided using the method of this embodiment ofthe invention.

The method then proceeds to step 264, where it ends until anotherencoded serial digital signal is received.

As illustrated in the preceding discussion and accompanying figures, themethod and apparatus of the present invention represent an improvementin the state of the art for optical communications of digital videosignals and associated methods. Various embodiments of the inventionprovide improved devices for reducing or eliminating pathological datapatterns from signals for transmission over optical communicationssystems. The improvements may be provided in one or more removableunits, such as a small form-factor pluggable (SFP) module compatiblewith existing optical communications equipment, at a reasonable cost.

While the exemplary embodiments described above have been chosenprimarily from the field of optical communication, one of skill in theart will appreciate that the principles of the invention are equallywell applied, and that the benefits of the present invention are equallywell realized in a wide variety of other communications systemsincluding, for example, electronic command and control systems. Further,while the invention has been described in detail in connection with thepresently preferred embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions, or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Accordingly, the invention is not to be seen as limited bythe foregoing description, but is only limited by the scope of theappended claims.

1. A method for transmission of serial digital video signals includingpathological data patterns over optical fibers comprising: (a)reversibly removing at least one pathological data pattern from a serialdigital video signal; (b) transmitting said serial digital video signalover at least one optical fiber pathway; and (c) restoring said at leastone pathological data pattern to said serial digital video signal.
 2. Amethod for communication of serial digital video signals includingpathological data patterns over optical fibers comprising: (a)reversibly removing at least one pathological data pattern from a serialdigital video signal; (b) receiving said serial digital video signalover at least one optical fiber pathway; and (c) restoring said at leastone pathological data pattern to said serial digital video signal.
 3. Amethod of processing digital signals, comprising: (a) receiving a firstserial data stream encoded according to a known standard, said firstencoded serial data stream including at least one pathological datapattern; (b) decoding said first serial data stream in accordance withsaid known standard; (c) converting said first serial data stream fromserial to parallel form to generate a parallel data stream; (d)dithering a least significant bit of said parallel data stream; (e)converting said dithered parallel data stream from parallel to serialform to generate a second serial data stream; and (f) encoding saidsecond serial data stream in accordance with said known standard togenerate a second encoded serial data stream in which said at least onepathological data pattern is not present.
 4. A method as in claim 3,further comprising: (a) receiving said second encoded serial datastream; (b) decoding said second encoded serial data stream inaccordance with said known standard to recover said second serial datastream; (c) converting said second serial data stream from serial toparallel form to recover said dithered parallel data stream; (d)un-dithering a least significant bit of said dithered parallel datastream to recover said parallel data stream; (e) converting saidparallel data stream from parallel to serial form to recover said firstserial data stream; and (f) encoding said first serial data stream inaccordance with said known standard to recover said first encoded serialdata stream including said at least one pathological data pattern.
 5. Amethod as in claim 3, wherein said known standard is SMPTE 259M.
 6. Amethod as in claim 3, wherein said known standard is SMPTE 292M.
 7. Amethod as in claim 4, wherein each of said decoding and encoding stepsincludes using a pseudo random sequence generator.
 8. A method as inclaim 3, wherein said dithering step includes using a pseudo randomsequence generator to dither said least significant bit of said paralleldata stream.
 9. A method as in claim 8, wherein said pseudo randomsequence generator includes a feedback shift register formed from Dflip-flops, feedback taps, and XOR gates.
 10. A method as in claim 8,wherein said pseudo random sequence generator has a characteristicpolynomial G(X)=X⁵+X²+1.
 11. A method as in claim 8, wherein said pseudorandom sequence generator has a characteristic polynomial G(X)=X⁵+X³+1.12. A method as in claim 3, wherein said first serial data streamincludes digital video data.
 13. An apparatus for removing pathologicaldata patterns from signals, comprising: (a) a decoder adapted to receivean encoded digital data signal in serial form, said decoder beingconfigured to reverse encoding previously applied in accordance with aknown data encoding standard; (b) a deserializer coupled to receive saiddigital data signal in serial form via said decoder and configured toconvert said digital data signal from serial to parallel form; (c) aditherer adapted to dither a least significant bit of said digital datasignal in parallel form, said ditherer configured to generate a dithereddigital data signal in parallel form; (d) a serializer coupled toreceive said dithered digital data signal in parallel form via saidditherer and configured to convert said dithered digital data signalfrom parallel to serial form; (e) an encoder coupled to receive saiddithered digital data signal in serial form via said serializer andconfigured to encode said dithered digital data signal in accordancewith said known data encoding standard.
 14. An apparatus as in claim 13,wherein said encoder and said decoder each include a pseudo randomsequence generator.
 15. An apparatus as in claim 13, wherein saiddeserializer includes a demultiplexer and said serializer includes amultiplexer.
 16. An apparatus as in claim 13, wherein said dithererincludes a pseudo random sequence generator with characteristicpolynomial G(X)=X⁵+X²+1.
 17. An apparatus as in claim 13, wherein saidditherer includes a pseudo random sequence generator with characteristicpolynomial G(X)=X⁵+X³+1.
 18. An apparatus as in claim 13, wherein saidditherer includes a feedback shift register formed from D flip-flops,feedback taps, and XOR gates.
 19. An apparatus as in claim 13, whereinsaid ditherer is adapted to add reversible noise to said digital datasignal.
 20. An apparatus as in claim 13, wherein said ditherer isadapted to add a reversible noise signal to one or more significant bitsof said digital data signal in parallel form.
 21. An apparatus as inclaim 13, wherein said apparatus is provided in a single removable unit.22. An apparatus as in claim 13, wherein said apparatus is provided in asmall form factor pluggable (SFP) module.
 23. An apparatus for restoringpathological data patterns previously removed from signals, comprising:(a) a decoder adapted to receive an encoded digital data signal inserial form, said decoder being configured to reverse encodingpreviously applied in accordance with a known data encoding standard;(b) a deserializer coupled to receive said digital data signal in serialform via said decoder and configured to convert said digital data signalfrom serial to parallel form; (c) an un-ditherer adapted to reversedithering previously applied to a least significant bit of said digitaldata signal in parallel form, said un-ditherer configured to generate anun-dithered digital data signal in parallel form; (d) a serializercoupled to receive said un-dithered digital data signal in parallel formvia said un-ditherer and configured to convert said un-dithered digitaldata signal from parallel to serial form; (e) an encoder coupled toreceive said un-dithered digital data signal in serial form via saidserializer and configured to encode said un-dithered digital data signalin accordance with said known data encoding standard.
 24. An apparatusas in claim 23, wherein said ditherer includes a pseudo random sequencegenerator with characteristic polynomial G(X)=X⁵+X²+1.
 25. An apparatusas in claim 23, wherein said ditherer is adapted to add reversible noiseto said digital data signal.
 26. An apparatus as in claim 23, whereinsaid apparatus is provided in a small form factor pluggable (SFP)module.
 27. A system for communication of serial digital video signals,comprising: (a) a transmitter device; (b) a receiver device; (c) a datacommunications link adapted to couple signals between said transmitterdevice and said receiver device; (d) wherein said transmitter deviceincludes at least one signal dithering device having a decoder adaptedto decode digital video signals in serial form in accordance with aknown encoding standard, a deserializer adapted to convert said digitalvideo signals from serial to parallel form, a ditherer adapted to applyreversible noise to said digital video signals in parallel form, aserializer adapted to convert said dithered digital video signals fromparallel to serial form, and an encoder adapted to encode said dithereddigital video signals in serial form in accordance with said knownencoding standard; and (e) wherein said receiver device includes atleast one signal un-dithering device having a decoder adapted to decodesaid dithered digital video signals in serial form in accordance withsaid known encoding standard, a deserializer adapted to convert saiddithered digital video signals from serial to parallel form, anun-ditherer adapted to reverse application of said reversible noise tosaid digital video signals in parallel form, a serializer adapted toconvert said un-dithered digital video signals from parallel to serialform, and an encoder adapted to encode said un-dithered digital videosignals in serial form in accordance with said known encoding standard.28. A system as in claim 27, further comprising: (a) a source of saiddigital video signals in serial form; (b) a source-side pathway adaptedto couple signals between said source and said transmitter device; (c) aconsumer of said un-dithered digital video signals in serial form; (d) aconsumer-side pathway adapted to couple signals between said receiverdevice and said consumer.
 29. A system as in claim 28, wherein at leastone of said transmitter device and said receiver device is included in aremovable unit adapted for use with a variety of communicationsequipment.
 30. A system as in claim 29, where said removable unitincludes a small form factor pluggable (SFP) module.
 31. A system as inclaim 27, wherein said transmitter device is functionally identical tosaid receiver device.
 32. A system as in claim 27, wherein saidtransmitter device is configured to receive said digital video signalsin serial form including at least one pathological data pattern, saidditherer is configured to reversibly remove said at least onepathological data pattern, said unditherer is configured to restore saidat least one pathological data pattern, and said receiver device isconfigured to provide said digital video signals in serial formincluding said at least one pathological data pattern.
 33. A system asin claim 27, wherein said known encoding standard is SMPTE 259M.
 34. Asystem as in claim 27, wherein said known encoding standard is SMPTE292M.
 35. A system as in claim 27, wherein said data communications linkincludes optical fiber communications devices.
 36. A system as in claim27, wherein said data communications link includes at least one DWDMbackbone.
 37. A system as in claim 27, wherein said data communicationslink includes AC-coupled optical communications devices.
 38. A system asin claim 27, wherein said data communications link includes free spaceoptics (FSO) communications devices.
 39. A system as in claim 27,wherein said data communications link includes physical layer switchingdevices.